Metal interconnects in a semiconductor device are typically formed by depositing a metal in an opening such as a via or trench in a dielectric layer on a substrate. As the width of interconnects has decreased in order to keep pace with a demand for higher performing devices, conventional SiO2 dielectric layers with a dielectric constant (k value) of about 4 are being replaced by materials with a k value of less than 3.5 and preferably less than 3. The lower k value in a dielectric layer helps to prevent crosstalk between metal wiring and reduces Rc delay, especially for technologies beyond the 130 nm node. Further reduction in k value can be accomplished by increasing the pore size or number of air pockets within a dielectric layer. However, porous dielectric layers are more susceptible to damage during a plasma ashing process such as one required to strip an adjacent photoresist layer during a rework step.
Frequently, a metal interconnect is formed above a conductive layer to provide an electrical pathway to another conductive layer within a stack of conductive layers. Copper is a preferred metal due to a lower resistivity than aluminum. However, copper may also be damaged by plasma ashing during a photoresist rework step. Other ashing steps such as those to remove an inert photoresist plug in a via during a dual damascene process or to strip a photoresist after pattern transfer through a dielectric layer can also damage an exposed copper surface. Therefore, a photoresist removal process that does not involve a plasma etch or ashing step is desirable to minimize damage to an underlying low k dielectric layer and to an exposed copper layer.
An improved photoresist removal process should also be able to strip a photoresist layer or an organic polymer layer that has been hardened by heating or by exposure to ultraviolet (UV) radiation. For example, an inert plug may be formed in a via in a dual damascene process to improve the process latitude of a subsequent photoresist patterning step that forms a trench above the via. However, the plug which is comprised of a hardened organic layer is difficult to remove by non-plasma methods because of its crosslinked character. Likewise, an anti-reflective coating (ARC) is often formed on a substrate just prior to coating a photoresist layer to improve the process latitude of the patterning step. An ARC also prevents trace amounts of contaminants in the underlying dielectric layer from interfering with a chemical reaction in exposed areas of the photoresist. When the ARC is a crosslinked organic polymer, the ARC is difficult to remove other than by a plasma ashing.
The process of reworking a photoresist may occur before or after a patterning step for various reasons including an incorrect exposure which produces an opening that has a width that is outside a specified limit. The stripping process may be required to remove an ARC at the same time. In any case, whether or not an ARC is used, the rework process has a tendency to alter the properties of a low k dielectric layer directly underneath an organic layer. For example, the dielectric constant of the low k dielectric layer may increase or the refractive index (real and imaginary components) can be changed so that the ARC refractive index is no longer matched to that of the dielectric layer. As a result, reflectivity off the low k dielectric layer is increased during a subsequent photoresist patterning step and process latitude is decreased. Therefore, a photoresist removal method should have a minimal effect on the properties of an underlying low k dielectric layer.
Ozone has been used to remove organic materials in an asher. In U.S. Pat. No. 5,677,113, a strip method involving ozone and UV radiation is described. U.S. Pat. No. 6,465,356 describes a strip method with ozone, heat, and UV exposure.
A method for stripping organic layers on a substrate with ozone and H2O in a gas phase is provided in U.S. Pat. No. 6,551,409. A rinse fluid consisting of 5% H2SO4 in H2O2 may also be applied.
In U.S. Pat. No. 5,911,837, an organic layer is stripped from a substrate in a tank of DI water by diffusing ozone into the solution and then rinsing with DI water at a higher temperature. A thin layer of organic solvent is then added on the water layer and acts as a drying solvent as the wafer is slowly pulled out of the water through the low vapor pressure solvent which is octane, decane, or a ketone.
The prior art includes stripping methods with an ozone treatment in a liquid phase. The use of ozone, NH4OH, and water is mentioned in U.S. Pat. No. 6,423,146. A method with ozone and HF is provided in U.S. Pat. No. 6,497,768. Optionally, photoresist and an ARC can be removed by a liquid spray comprised of ozone, NH4OH, and water. An apparatus for performing the aforementioned spray process is provided in U.S. Pat. No. 6,273,108.
Yet another method of removing an organic layer with ozone is mentioned in U.S. Pat. No. 6,242,165 where supercritical CO2 is employed as a carrier solvent for ozone. However, no reaction conditions are given that teach or suggest a way of using the CO2/O3 combination effectively since SO3 is described as the preferred oxidizer.